I write fanmail to my favourites, not just mangakas (Japanese or otherwise) -- birthday cards, whatever. I have not received any from the Japanese though. But who knows, you just might get lucky, right?

PS: When I was younger, I used to write to a lot of my favorite NBA players. I got a poster from Grant Hill.

Old thread, but just wanted to say if theres a animecon or other simular things in your area you can also check or ask if they are interesteed in inviting your favorite mangaka there. I meet a few mangakas that way.ask them to do drawing lesson can be fun.